I put "Your mum must go to hospital" and got a message 'You need the article "the" here.'
My english seems fine to me so normally I'd just report it but because of that message I'm wondering if there is something about that sentence in Vietnamese that actually requires the article 'the' between 'to' and 'hospital'?
I'm not sure about what you're saying. Maybe that's what you'd say in Ireland or the UK (or even Australia). However in Canada and the US, we would say, "Your mom must go to the hospital." That's how we would say it here, so your phrasing sounds a little weird to us. Maybe the DL team knows American or Canadian English. At least, they recognized "mum". Maybe you should report it and say it's how you would phrase it in your dialect.